The invention is directed to the field of diagnostic X-ray imaging apparatus, and more particularly to a novel and improved method and apparatus for obtaining a selectable contrast image in a diagnostic X-ray exposure.
Diagnostic X-ray imaging apparatus has traditionally provided a program and technique, often presented on a chart, which allows a selection of three key factors prior to each exposure in an effort to optimize the levels of contrast in each exposure. These factors are (1) KVP - the peak kilovoltage (KV) applied to the X-ray tube during exposure; (2) MA - the milliamperes current forced through the X-ray tube in response to the selected KVP; and (3) time - the number of seconds, from on the order of tens of milliseconds up to on the order of ten seconds of duration of the exposure. More recently, the combined milliamperes and seconds are being combined in a term MAS, which refers to the multiplication product of the milliamperes times the seconds.
The KVP has two effects on the diagnostic value or exposure of the film. The peak KV value determines the penetrating power of the X-ray beam and also affects film-blackening. The MA or MAS determines the intensity of the beam, with total blackening of the film being directly proportional to the time of exposure.
A proper selection of these two (KVP and MAS) factors will produce an exposure of the film in varying levels of black and white contrast, forming an image of all parts of the body through which the X-rays have passed. That is, the various gradations of black and white exposure levels on the film will create a picture or image of the various body parts. This gradation may extend from an almost total black exposure for areas outside of the body part being imaged to a clear and virtually unexposed transparent film area for those body parts which are so dense that they totally absorb all X-rays that enter. In between the total black and clear or transparent areas, the gradations of film density provide diagnostic details in the form of variations of film density or black/white contrast levels.
In most X-ray generators heretofore, the KVP is determined by the voltage to the primary side of a high voltage generator circuit (allowing for voltage drops due to the regulation of the system). The earliest systems were single-phase, full-wave rectified, essentially giving a KVP waveform which converts a 60 Hz sine wave signal to two positive half-waves per cycle and transforms the voltage levels to the desired KVP voltage. Regardless of the KVP selected, the radiation exposure spectrum contained energies at all wavelengths from the lowest usable KV level (on the order of 30-40 KV) up to the KVP level selected by the operator. Hence as the KV level rises sinusoidally in these older systems, it first produces contrast on the exposure on the softer and less dense areas of the body part being imaged and continues to increase penetration into denser areas progressively. This relatively broad band of energy therefore permitted the X-ray film to display a relatively broad range of contrast for a given KVP setting. The greater the contrast of a film exposure, the easier it is, in theory, to visualize small density differences. As the KVP setting is increased, the range of contrast tends to be reduced, since it is the amount of relatively low level energy, produced by the lower KV levels, that determines the contrast range.
However, the foregoing single-phase system is highly inefficient by today's standards, and delivers an excessive amount of X-ray energy to the patient during the exposure. Later X-ray systems developed a three-phase technology designed to eliminate this waste of energy and excessive exposure. These three phase X-ray systems produced a waveform with very little energy at the lower kilovoltages, which greatly improve generator efficiency as compared to single phase. That is, instead of two sine waves per cycle, there were now six pulses interwoven in a mesh that eliminated much of the wasted energy in a single-phase system. However, the lowest KV level on any exposure was now only on the order of eighty percent of the peak KV value for the exposure. This resulted in a reduction of contrast in the exposure which was accepted as a trade-off for the higher efficiency. This means that smaller gradations of density are harder to identify in the exposed film, which may make diagnosis more difficult. Finally, with the development of high frequency X-ray generators, exposures are made at an almost constant kilovoltage level with very little "soft" radiation. In such systems, the efficiency is greatly improved by the use of high frequency switching which provides precise timing, precise duplication of exposures from one exposure to the next, and shortened exposure times. In high frequency generators, the kilovoltage at the generator is monitored and compared to a selectable, fixed reference voltage. Any resultant difference voltage is used to directly regulate the kilovoltage. As the reference voltage is changed by the operator, the kilovoltage changes correspondingly to a new level; that is, in a closed-loop type of system.
However, each improvement has further reduced the proportion of the lower KV levels to the selected KVP and therefore reduced the overall contrast of the image. However, because these changes have taken place over a long period of time, this gradual reduction in contrast has been generally accepted by diagnosticians.